This invention relates to a process for forming ceramic powders with fine nanosize particles.
Nanosize powders are generally considered to be powders having very fine particles in the nanometer range, i.e., less than a few nanometers, e.g., 100 nanometers or less, usually 10 nanometers or less.
Nanosize powders have numerous applications such as catalysts, electrocatalysts, catalyst supports, electrodes, active powders for the fabrication of dense bodies, semiconductors for energy storage, photovoltaics, ultrafine magnetic materials for information storage, environmental clean-up as destructive adsorbents, water purification, information storage, and optical computers, to name a few. Some of the numerous examples include the following: nanosize.(3 to 4 nm) platinum for oxygen reduction in acid electrolytes, many metallic powders made by precipitation in aqueous and non-aqueous media for alloy fabrication and for catalysis, nanosize iron oxide catalyst for coal liquefaction, nanosize iron oxide particles for magnetic applications, tetragonal zirconia powder by a hydrothermal treatment at high pressures for structural applications, carbides and nitrides using non-aqueous media, nanosize BaTiO3 by a gas-condensation process, etc. Many oxides have potential applications as nanosize powders. These include: CeO(2-x) for catalytic reduction of SO2,xcex3-alumina as a catalyst support and for enhancing ionic conductivity of lithium iodide, V2O5 as a catalyst for NOx reduction, and etc. Several processes currently used for the synthesis of nanosize powders include:(1) Gas-phase condensation, (2) Mechanical milling, (3) Thermal crystallization, (4) Chemical precipitation, (5) Sol-gel processing, (6) Aerosol spray pyrolysis, and etc.
In gas-phase condensation, evaporation of precursors and their interaction with an inert gas leads to loss of kinetic energy, and homogeneous nucleation of nanosize powders occurs in a supersaturated vapor. Nanocrystalline powders of TiO2, Li2O-doped MgO, CeO2, Y-doped ZrO2, etc. have been produced by gas-phase condensation. Aerosol spray pyrolysis has been used to synthesize BaFe12O19, Fe2O3 among some other materials. High-energy mechanical milling is used extensively to produce nanostructured materials, especially when large quantities of materials are required. Very fine particles of nickel-aluminum alloy, Fexe2x80x94Coxe2x80x94Nixe2x80x94Si alloys, Nixe2x80x94Mo alloys, for example, have been produced by mechanical milling. Contamination by the milling process, however, is a shortcoming of this process. Also, although very fine (nm size) particles can be made, agglomeration is a problem leading to cluster sizes in the micron range.
Chemical coprecipitation has received considerable attention for the synthesis of nanosize powders. Metallic as well as ceramic powders can be made by a careful control of chemistry. Alkali metal borohydride, MBH4 where M is an alkali metal, for example, has been used as a reducing agent in aqueous media for the synthesis of metallic powders. Similarly, hydroorganoborates of the general formula MHv(BR3) or MHv[BRn(ORxe2x80x2)3-n]v where M is an alkali or alkaline earth metal, v=1, 2, and R, Rxe2x80x2 are alkyl or aryl groups have been used as reducing and precipitating agents. It is important to control pH and ionic strength in aqueous media to prevent Ostwald ripening. In the synthesis of nanosize iron oxide, for example, it has been shown that the higher the pH and the higher the ionic strength, the smaller is the size of nanosize Fe3O4 particles.
In most methods for the synthesis of nanosize powders, two issues are particularly important; (1) the formation of fine, uniform size particles, and (2) the prevention of agglomeration. Nanoparticles of a uniform size can in principle be formed by carefully controlling nucleation and growth. Often, a variety of encapsulating methods are necessary to control the size of nanoparticles.
Agglomeration is often the result of Van der Waal""s forces. The adverse effect of agglomeration on the sintering behavior of ceramic powders is well documented. Even in catalysis, the need for dispersed powders is well known. Often, supercritical drying can be used to obtain nonagglomerated powders. In liquid media, agglomeration can be suppressed through steric hindrance or through the manipulation of electrostatic interactions. The latter in polar liquids can be achieved by changing the pH and the ionic strength of the solution. Many techniques involve the use of surfactants. Often the powders which are nonagglomerated and well dispersed in a liquid, tend to agglomerate during the drying stage. Fortunately, methods such as slip-casting, gel-casting, pressure slip casting can be used to achieve powder compaction in a wet state. Such has been demonstrated using submicron ceramic powders.
With the exception of milling, all the above methods are based on molecular synthesis of nanoparticles wherein the particles are built-up by atom-by-atom, or molecule-by-molecule, addition. Even in processes based on the decomposition of metal carbonyls, the growth of particles occurs by a layer-by-layer addition of atoms. As a result, a control of nucleation and growth is necessary to ensure the formation of nanosize particles. This often requires a very precise and difficult control of the reaction system, which renders the manufacture of the nanosize powder in large quantities impractical or impossible. In addition, the molecular synthesis processes are costly because of the relatively large capital expenditures required for the equipment to control the formation of only a small quantity of nanosize product.
It is, therefore, an object of the invention to provide method for the formation of nanosize powders that is easy to implement on an industrial scale and in relatively inexpensive when compared to molecular synthesis methods.
Another object of the invention is to provide a method in which nanosize powders are formed by a process other than precipitation or deposition from solutions, thus eliminating the possibility of unwanted deposition and growth of the nanosize powders.
Another object of the invention is to provide a method which forms nanosize powders that have a reduced tendency to agglomerate.
Another object of the invention is to provide a method for the formation of nanosize powders that can be applied to forming a variety of powder compositions.
Further objects of the invention will become evident in the description below.
In order to overcome the problems associated with molecular synthesis and milling to form nanosize powders, the present invention presents an alternative approach for the synthesis of nanosize powders. In the present invention, a precursor inorganic compound is formed from which the unwanted component is leached away so that a fine, nanosize powder is left as a residue. Thus, the present invention is based on molecular decomposition, rather than molecular synthesis, or deposition.
As discussed above, one of the problems with many methods of synthesis of nanosize powders is that often it is difficult to synthesize large quantities of materials. By contrast, the present invention is suitable for making large quantities of nanosize powders of a number of materials.
In summary, the present invention is a process for forming nanosize powders . The process comprises:
forming a precursor ceramic material comprising a fugitive constituent and a non-soluble constituent in a single phase;
contacting the precursor material a selective solvent to form a solution of the fugitive constituent and a residue of the non-soluble constituent, the precursor sufficiently reactive with the solvent to form the solution of the fugitive constituent in the solvent and the residue of the non-soluble constituent the precursor material and the non-soluble residue sufficiently insoluble in the solvent such that there is insufficient precursor material and non-soluble residue in solution to deposit and precipitate upon the residue of the non-soluble-constituent, the fugitive constituent being sufficiently soluble in the solvent such that the precursor reacts with the solvent to form a solution of the fugitive constituent without precipitation and deposition of fugitive constituent upon the residue of the non-soluble constituent in the form of nanosize particles;
removing the selective solvent solution from the residue to form a nanosize powder having the same chemical composition as the non-soluble constituent.
The precursor material should be insoluble as the precursor material in the solvent. One of the objects of the present invention is to prevent deposition or precipitation of dissolved materials upon the nanosize particles that are formed from the non-soluble residue freed of the fugitive constituent. Deposition or dissolved precursor material will not only contaminate the residue, but possibly result in particles that are too large. In the present invention, an object is to prevent, as much as possible, deposition of dissolved materials and the resulting growth of existing crystals. Unlike prior-art methods where small crystals are crystallized from solution, it is practical in the present invention to inhibit the crystallization and precipitation process altogether, as crystallization is not required to form initial nanosize crystals. Accordingly, as further illustrated below, a precursor is chosen such that precipitation of any material upon the nanosize particles is essentially avoided.
Another requirement of the precursor material is that it be reactive with the solvent. Since the precursor is insoluble, the precursor composition does not dissolve, but it does react to selectively remove the fugitive constituent, leaving a freed non-soluble constituent. The fugitive constituent is sufficiently soluble that it will not precipitate from the solution to contaminate and grow the particles of the non-soluble constitituent. The non-soluble constituent is essentially insoluble to prevent dissolved material dissolving into the solution, there thereafter precipitating upon and growing the nanosize particles of the non-soluble material formed by removal of the fugitive constituent. Basically, the invention involves a balance between preventing precursor and non-soluble constituent from forming a solution so that it cannot redeposit and grow crystals, and removing the fugitive constituent, that is sufficiently soluble in the solution that it will not precipitate from the solution to grow crystals.
For example, BaCeO3 does not dissolve in water, but reacts with water to form a solution of the Ba (as Ba(OH)2). On the contrary, NaAlO2 is soluble in water, and cannot be used as a suitable precursor for the present invention.
In order that the non-soluble constituent freed from the fugitive constituent form nanosize particles, the precursor should be a single phase material, i.e., where the differentiation between the non-soluble and the fugitive constituents is on a molecular level. Accordingly, the precursor exists as a compound existing is the form of a large molecule, or as an alloy. For ceramics, an example of a precursor material is a mixed oxide with at least two cations, the metal oxide of the first cation being soluble to function as the fugitive constituent, and the oxide of the second being non-soluble to function as the non-soluble constituent For forming, metal nanosize powders, the precursor can exist, for example, as an alloy or an intermetallic compound.
The solvent is selected to react with the precursor and have the solubility properties as described above. For ceramics, a preferred solvent is water, but non-aqueous solvents may be required to suppress the solubilities of the non-soluble residue and/or the precursor. Usually the solvent is polar, as the fugitive constituent, which must be solubilized, is often a polar composition. Typically, suitable solvents for forming a ceramic nanosize powder are polar liquids that solubilize selected ceramic oxides, which selected oxides in the process of the invention are the fugitive constituents. As further described below, preferred solvents are water and acids. For metallic nanosize powders, the selective solvent is typically an acid that reacts with or dissolves the fugitive metal, and not the non-soluble metal. In place of acids, an acid gas such as SO3, N2O5, CO2 or HCl, may be used to react with and remove the fugitive constituent.
Any suitable process may be used for forming a precursor is contemplated.
For example, where the precursor is a mixed ceramic subjecting a powder mixture to a suitable high-temperature treatment is suitable. Precursors for metallic powders are likewise formed by any suitable process to produce a suitable alloy or intermetallic compound, such as alloying methods or powder metallurgy.
The solvent is then removed from the remaining non-soluble constituent. With the fugitive constituent removed, the non-soluble constituent assumes the form of nanosize particles. The solvent is removed by conventional methods, including, but not limited to settling, centrifugation, filtering, air drying, or a combination or the above. The solvent, may also be removed by replacing it with another liquid, such as replacing at aqueous solvent with an alcohol, and then separating the powder from the replacement liquid. Using a replacement liquid may be desirable to inhibit agglomeration of the powder during drying.
The product is chemically the same as the non-soluble constituent. The process of the invention is particularly suitable for synthesizing nanosize powders of Al2O3, CeO2, ZrO2, TiO2, V2O5, rare earth (RE) oxide-doped CeO2 and RE- or Y2O3-doped ZrO2.